Latest content added for UNT Digital Library Partner: UNT Librarieshttp://digital.library.unt.edu/explore/partners/UNT/browse/?fq=str_degree_discipline:Physics&start=40&fq=untl_collection:UNTETD2014-03-24T20:07:29-05:00UNT LibrariesThis is a custom feed for browsing UNT Digital Library Partner: UNT LibrariesL-Shell X-Ray Production Cross Sections for ₂₀Ca, ₂₆Fe, ₂₈Ni, ₂₉Cu, ₃₀Zn, ₃₁Ga, and ₃₂Ge by Hydrogen, Helium, and Lithium Ions2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277620/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277620/"><img alt="L-Shell X-Ray Production Cross Sections for ₂₀Ca, ₂₆Fe, ₂₈Ni, ₂₉Cu, ₃₀Zn, ₃₁Ga, and ₃₂Ge by Hydrogen, Helium, and Lithium Ions" title="L-Shell X-Ray Production Cross Sections for ₂₀Ca, ₂₆Fe, ₂₈Ni, ₂₉Cu, ₃₀Zn, ₃₁Ga, and ₃₂Ge by Hydrogen, Helium, and Lithium Ions" src="http://digital.library.unt.edu/ark:/67531/metadc277620/thumbnail/"/></a></p><p>L-shell x-ray production cross sections are presented for Fe, Ni, Cu, Zn, Ga, and Ge by 0.5- to 5.0-MeV protons and by 0.5- to 8.0-MeV helium ions and Ca, Fe, Ni, Cu, and Ge by 0.75- to 4.5-MeV lithium ions. These measurements are compared to the first Born theory and the perturbed-stationary- state theory with energy-loss, Coulomb deflection, and relativistic corrections (ECPSSR). The results are also compared to previous experimental investigations. The high precision x-ray measurements were performed with a windowless Si(Li) detector. The efficiency of the detector was determined by the use of thin target atomic-field bremsstrahlung produced by 66.5 keV electrons. The measured bremsstrahlung spectra were compared to theoretical bremsstrahlung distributions in order to obtain an efficiency versus energy curve. The targets for the measurement were manufactured by the vacuum evaporation of the target element onto thin foils of carbon. Impurities in the carbon caused interferences inthe L-shell x-ray peaks. Special cleansing procedures were developed that reduced the impurity concentrations in the carbon foil, making the use of less than 5 μg/cm^2 targets possible. The first Born theory is seen to greatly overpredict the data at low ion energies. The ECPSSR theory matches the data very well at the high energy region. At low energies, while fitting the data much more closely than the first Born theory, the ECPSSR theory does not accurately predict the trend of the data. This is probably due to the onset of molecular-orbital effects, a mechanism not accounted for in the ECPSSR theory.</p>Synchronous Chaos, Chaotic Walks, and Characterization of Chaotic States by Lyapunov Spectra2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277794/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277794/"><img alt="Synchronous Chaos, Chaotic Walks, and Characterization of Chaotic States by Lyapunov Spectra" title="Synchronous Chaos, Chaotic Walks, and Characterization of Chaotic States by Lyapunov Spectra" src="http://digital.library.unt.edu/ark:/67531/metadc277794/thumbnail/"/></a></p><p>Four aspects of the dynamics of continuous-time dynamical systems are studied in this work. The relationship between the Lyapunov exponents of the original system and the Lyapunov exponents of induced Poincare maps is examined. The behavior of these Poincare maps as discriminators of chaos from noise is explored, and the possible Poissonian statistics generated at rarely visited surfaces are studied.</p>An Experimental Study of Collision Broadening of some Excited Rotational States of the Bending Vibration of Methyl Cyanide2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc278369/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc278369/"><img alt="An Experimental Study of Collision Broadening of some Excited Rotational States of the Bending Vibration of Methyl Cyanide" title="An Experimental Study of Collision Broadening of some Excited Rotational States of the Bending Vibration of Methyl Cyanide" src="http://digital.library.unt.edu/ark:/67531/metadc278369/thumbnail/"/></a></p><p>A double modulation microwave spectrometer is used to evaluate the linewidth parameters for some excited rotational components in the bending vibration v_8 of 13CH3 13C 15N and 13CH3C15N isotopomers of methyl cyanide. The linewidth parameters for self-broadening of the ΔJ=2←1 rotational components for the ground v_8 , 1v_8, and the 2v_8 vibrations were determined over a pressure range of 1 to 13 mtorr and at a temperature of 300 K. The double modulation technique is used to explore the high eighth derivative of the line shape profile of the spectral line. This technique proved to give good signal-to-noise ratios and enabled the recovery of weak signals. An experimental method is developed to correct for source modulation broadening. The tests of the ratios of the two inner peak's separation of the eighth derivative of the line showed that they were up to 95% similar to those for a Lorentzian line shape function. The line shapes were assumed to be Lorentzian for the theoretical analysis of the derivative profiles and comparisons were made between experiment and theory on this basis. Dipole moments for vibrationally excited states were calculated from linewidth parameters and show systematic decrease with the increase of excitation. Impact parameters were calculated using the "hard sphere" model of the kinetic theory of gases. The results were many times larger than the size of the molecule itself. This suggests that the dominant interaction is a long range dipole-dipole force interaction.</p>Short-Period Transient Grating Measurement of Perpendicular Transport in GaAs/AlGaAs Multiple Quantum Wells2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277907/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277907/"><img alt="Short-Period Transient Grating Measurement of Perpendicular Transport in GaAs/AlGaAs Multiple Quantum Wells" title="Short-Period Transient Grating Measurement of Perpendicular Transport in GaAs/AlGaAs Multiple Quantum Wells" src="http://digital.library.unt.edu/ark:/67531/metadc277907/thumbnail/"/></a></p><p>In this thesis the author describes the use of transient grating techniques to study the transport of electrons and holes perpendicular to the layers of a GaAs/AlGaAs multiple quantum well (MQW).</p>Nonlinear Dynamics of Semiconductor Device Circuits and Characterization of Deep Energy Levels in HgCdTe by Using Magneto-Optical Spectroscopy2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc278165/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc278165/"><img alt="Nonlinear Dynamics of Semiconductor Device Circuits and Characterization of Deep Energy Levels in HgCdTe by Using Magneto-Optical Spectroscopy" title="Nonlinear Dynamics of Semiconductor Device Circuits and Characterization of Deep Energy Levels in HgCdTe by Using Magneto-Optical Spectroscopy" src="http://digital.library.unt.edu/ark:/67531/metadc278165/thumbnail/"/></a></p><p>The nonlinear dynamics of three physical systems has been investigated. Diode resonator systems are experimentally shown to display a period doubling route to chaos, quasiperiodic states, periodic locking states, and Hopf bifurcation to chaos. Particularly, the transition from quasiperiodic states to chaos in line-coupled systems agrees well with the Curry-Yorke model. The SPICE program has been modified to give realistic models for the diode resonator systems.</p>Transport Processes in Synchrotrons2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277803/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277803/"><img alt="Transport Processes in Synchrotrons" title="Transport Processes in Synchrotrons" src="http://digital.library.unt.edu/ark:/67531/metadc277803/thumbnail/"/></a></p><p>This thesis examines the evolution of beams in synchrotrons. Following an introduction to accelerator physics in Chapter 1, in Chapter 2 I describe the Fermilab E778 'diffusion' experiment. Families of sextupoles were powered to drive the 2/5 resonance, and a beam was then kicked to populate a nonlinear region of the transverse phase space. The beam was then observed over periods of approximately 30 minutes for a variety of kick amplitudes and physical apertures. In Chapter 3 comments about the analytic treatment of such systems are discussed, including the assumptions inherent in the conventional treatment. I motivate my use of a simplified model in Chapter 4 after examining common computational methods. Deriving the model from the formalism of traditional accelerator physics, I discuss its implementation on a massively parallel computer, the Intel iPSC/860 hypercube, and examine the performance of this algorithm in detail. Using the simple model to perform the numerical experiment equivalent to E778 is the subject of Chapter 5. I derive the parameters needed for the simple model based upon the physical experiment. Both three dimensional cases and cases with reduced dimensionality are run. From power supply ripple data and an electrical model of the magnet string, I compute tune modulation depths, and a subset of these are run. I conclude that tune modulation from power supply ripple is not a significant source of transport for this system. In Chapter 6, the intensities of the beams are used to compare the experimental and numerical runs, using both exponential and algebraic decays, and the algebraic form is seen to provide a better fit. The agreement between numerical and experimental results is best for fully three-dimensional runs, but the numerical results show slower decay than the experimental. Individual particles are examined, whose motion consists of stochastic motion interspersed with regular motion, suggestive of a Continuous Time Random Walk process. A pausing time distribution is extracted which is algebraic in time, which is consistent with dispersive transport observed elsewhere.</p>Nonlinear Optical Absorption and Refraction Study of Metallophthalocyanine Dyes2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc278287/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc278287/"><img alt="Nonlinear Optical Absorption and Refraction Study of Metallophthalocyanine Dyes" title="Nonlinear Optical Absorption and Refraction Study of Metallophthalocyanine Dyes" src="http://digital.library.unt.edu/ark:/67531/metadc278287/thumbnail/"/></a></p><p>This dissertation deals with the characterization of the nonlinear absorption and refraction of two representative metallophthalocyanine dyes: chloro aluminum phthalocyanine dissolved in methanol, referred to as CAP, and a silicon naphthalocyanine derivative dissolved in toluene, referred to as SiNc. Using the Z-scan technique, the experiments are performed on both the picosecond and nanosecond timescales at a wavelength of 0.532 μm.</p>The Fractal Stochastic Point Process Model of Molecular Evolution and the Multiplicative Evolution Statistical Hypothesis2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277827/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277827/"><img alt="The Fractal Stochastic Point Process Model of Molecular Evolution and the Multiplicative Evolution Statistical Hypothesis" title="The Fractal Stochastic Point Process Model of Molecular Evolution and the Multiplicative Evolution Statistical Hypothesis" src="http://digital.library.unt.edu/ark:/67531/metadc277827/thumbnail/"/></a></p><p>A fractal stochastic point process (FSPP) is used to model molecular evolution in agreement with the relationship between the variance and mean numbers of synonymous and nonsynonymous substitutions in mammals. Like other episodic models such as the doubly stochastic Poisson process, this model accounts for the large variances observed in amino acid substitution rates, but unlike other models, it also accounts for the results of Ohta's (1995) analysis of synonymous and nonsynonymous substitutions in mammalian genes. That analysis yields a power-law increase in the index of dispersion and an inverse power-law decrease in the coefficient of variation with the mean number of substitutions, as predicted by the FSPP model but not by the doubly stochastic Poisson model. This result is compatible with the selection theory of evolution and the nearly-neutral theory of evolution.</p>A Study of Some Biological Effects of Non-Ionizing Electromagnetic Radiation2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc278105/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc278105/"><img alt="A Study of Some Biological Effects of Non-Ionizing Electromagnetic Radiation" title="A Study of Some Biological Effects of Non-Ionizing Electromagnetic Radiation" src="http://digital.library.unt.edu/ark:/67531/metadc278105/thumbnail/"/></a></p><p>The experimental studies of this work were done using a microwave cavity spectrometer, Escherichia coli (E-coli) bacteria, and other peripheral equipment. The experiment consists of two steps. First, a general survey of frequencies from 8 GHz to 12 GHz was made. Second, a detailed experiment for specific frequencies selected from the first survey were further studied. Interesting frequency dependent results, such as unusually higher growing or killing rates of E-coli at some frequencies, were found. It is also concluded that some results are genetic, that is, the 2nd, and 3rd subcultures showed the same growing status as the 1st cultures.</p>Microstructural Studies of Dental Amalgams Using Analytical Transmission Electron Microscopy2014-03-24T20:07:29-05:00http://digital.library.unt.edu/ark:/67531/metadc277884/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc277884/"><img alt="Microstructural Studies of Dental Amalgams Using Analytical Transmission Electron Microscopy" title="Microstructural Studies of Dental Amalgams Using Analytical Transmission Electron Microscopy" src="http://digital.library.unt.edu/ark:/67531/metadc277884/thumbnail/"/></a></p><p>Dental amalgams have been used for centuries as major restorative materials for decaying teeth. Amalgams are prepared by mixing alloy particles which contain Ag, Sn, and Cu as the major constituent elements with liquid Hg. The study of microstructure is essential in understanding the setting reactions and improving the properties of amalgams. Until the work reported in this dissertation, optical microscopy (OM), scanning electron microscopy (SEM), and x-ray diffractometry (XRD) were used commonly to analyze amalgam microstructures. No previous systematic transmission electron microscopy (TEM) study has been performed due to sample preparation difficulties and composite structure of dental amalgams. The goal of this research was to carry out detailed microstructural and compositional studies of dental amalgams. This was accomplished using the enhanced spatial resolution of the TEM and its associated microanalytical techniques, namely, scanning transmission electron microscopy (STEM), x-ray energy dispersive spectroscopy (XEDS) and micro-microdiffraction (μμD). A new method was developed for thinning amalgam samples to electron transparency using the "wedge technique." Velvalloy, a low-Cu amalgam, and Tytin, a high-Cu amalgam, were the two amalgams characterized. Velvalloy is composed of a Ag₂Hg₃ (γ₁)/HgSn₇₋₉ (γ₂) matrix surrounding unreacted Ag₃Sn (γ) particles. In addition, hitherto uncharacterized reaction layers between Ag₃Sn(γ)/Ag₂Hg₃ (γ₂) and Ag₂Hg₃ (γ₁)/HgSn₇₋₉ (γ₂) were observed and analyzed. An Ag-Hg-Sn (β₁) phase was clearly identified for the first time. In Tytin, the matrix consists of Ag₂Hg₃ (γ₁) grains. Fine precipitates of Cu₆Sn₅ (η') are embedded inside the γ₁ and at the grain boundaries. These precipitates are responsible for the improved creep resistance of Tytin compared to Velvalloy. The additional Cu has completely eliminated the γ₂ phase which is the weakest component of amalgams. Ag-Hg-Sn (β₁) and large grains of Cu₆Sn₅ (η') are found adjacent to the unreacted alloy particles. Tytin alloy particles contain Cu₃Sn (ε) precipitates in a matrix of Ag₃Sn (γ) and Ag₄Sn (β). SEM was used to correlate the TEM findings in the context of the general microstructure. The results are in good agreement with those published in the literature. The microstructural details reported here, many of which were not previously available, will help provide insight into the deformation mechanisms of dental amalgams.</p>